46 MASSACHUSETTS HORTICULTURAL SOCIETY. 



performed for the flowering plants by seeds, although no spore is 

 the exact equivalent of a seed. The spores in question are known 

 as the summer spores of the mildew. When they are sown in water 

 at a warm temperature remarkable changes occur in a few hours. 

 First, the living matter in a spore becomes divided into several 

 masses, which escape through an opening in the membrane at one 

 end of the spore. Each of these masses is furnished with two hair- 

 like lashes, by whose vibration it is propelled rapidl}' through the 

 water. From their remarkable activity, which recalls that of 

 animals, these bodies are called zoospores. After a short time 

 their motion ceases, the threads disappear, and the mass germinates 

 by producing a tube from its side. If this germination occurs on 

 the surface of a grape leaf, the tube pushes through a breathing- 

 pore or bores through the surface cells to the interior of the leaf, 

 where it produces the characteristic absorbing organs of the fungus, 

 and a new parasite is established. If the germination takes place 

 elsewhere than on a grape leaf, the tube soon ceases to grow and 

 then dies. If, on the other hand, one of the spores fails to find 

 the moisture and warmth necessary to its further development it 

 soon dies. 



We may notice here two points of general application. The 

 species of parasitic fungi are limited, as a rule, each to a single or 

 to a few closely related species of higher plants as hosts, and are 

 incapable of development on others. The closely restricted condi- 

 tions of successful germination of the spores of a fungus, demand 

 that to secure the perpetuation of the species a vast number of 

 spores shall be produced, that the enormous waste may not be 

 fatal. As the parasite becomes mature the leaf becomes drier 

 and more shrunken, and the fungus begins to provide for tiding 

 over the approaching season of conditions unfavorable for growth. 

 This is accomplished by the formation of ivinter spores^ in whose 

 production a true sexuality is shown. They differ from the sum- 

 mer spores in having very thick walls and in germinating only 

 after a period of rest, and are produced from the threads of the 

 fungus within the leaf. The end of a fungus-thread swells up into 

 a rounded body containing a large amount of living matter, which 

 becomes aggregated into a dense globular mass. The end of 

 another branch becomes irregularly swollen, though of a much 

 smaller size, and is closely applied to the surface of the larger 



